Cathode ray tube focusing arrangements



- Feb. 17, 1970 R. KIRKHAM CATHODE RAY TUBE FOCUSING ARRANGEMENTS Filed May 1'7. 1968 m I m m w. u v M Av mm W I 3,496,408 CATHODE RAY TUBE FOCUSING ARRANGEMENTS Raymond Kirkham, Welwyn Garden City, England, as-

signor to The Rank Organisation Limited, London, England, a British company Filed May 17, 1968, Ser. No. 730,141 Int. Cl. H01j 29/56 U.S. Cl. 315-31 9 Claims ABSTRACT OF THE DISCLOSURE Electron beam focus control apparatus in which a variable signal is produced from signals representative of the position of the beam in a cathode ray tube requiring different degrees of focusing at difierent parts of its face. The variable signal is produced in a non-linear amplifier from signals from scanning and/ or deflection circuits and is suitable for application to focusing plates or coils of the tube.

This invention relates to arrangements for focusing an electron beam in a cathode ray tube.

In a cathode ray tube, an electron beam is produced by a gun disposed at one end of an evacuated envelope, the beam being directed and accelerated onto an internally phosphor coated face plate at an opposite end of the envelope. In most applications the electron beam is focused onto the phosphor coated face plate, to produce a sharply defined image either of an aperture in one of the gun electrodes or of a portion of the beam within the gun. Such focusing may be produced by magnetic or by electrostatic fields which deflect the beam about a predetermined point so that a focus condition is produced.

With a constant focus field, this focus condition is maintained only if the separation between the tube face and this predetermined point about which the beam is deflected by the focusing field remains substantially constant. Accordingly in many tubes, for example in flat faced tubes, in which this separation varies with position on the tube face, a sharply focused condition of the beam cannot be maintained as the beam is scanned across the tube face.

It has been proposed to prevent such defocusing by using current or voltage driven focusing elements and by applying to such focusing elements, a driving voltage or current, the amplitude of which is both time dependent and is synchronised with the scan of the beam across the tube face. The time dependence of the driving voltage or current is arranged such that for any beam position, the amplitude is effective to produce and maintain sharp focusing of the beam. The time dependence of the driving voltage or current thus predicts the beam position on the tube face and ensures that focus conditions are maintained by adjusting the amplitude on the basis of such prediction.

This method of maintaining focus, however, suffers from the disadvantage that the time dependence of the driving voltage or current must be altered as the scanning amplitude and speed is altered and, in addition, is unable therefore to take into account variations in beam position brought about by means other than scanning, which cannot be predicted.

It is an object of the invention to provide focus control signals that are independent of time.

According to the invention there is provided apparatus for controlling focusing of an electron beam produced in, and directed onto the face of, a cathrode ray tube and requiring different degrees of focusing at different parts of said face, comprising electric circuit means responsive to 3,496,408 Patented Feb. 17, 1970 signals representative of the position of said beam within said tube to produce an output signal representative of the degree of focusing required for that position of said beam so as to maintain sharp focusing of said beam irrespective of its position when that output signal is applied to focusing means associated with said tube.

The focusing element may be voltage driven focusing plates within the tube or current driven coil or coils outside the tube.

In the case where the only beam deflection is that produced by scanning, the electric circuit means conveniently are arranged to be responsive to signals derived from the output of the scanning circuit or circuits, for example the frame and line scanning generators in television receiving or transmitting apparatus.

In the case where the beam is deflected additionally to scanning, for example in an oscilloscope for waveform display, the electric circuit means are arranged to be responsive to signals derived from a scanning circuit, if any, and from those means arranged to deflect the beam independently of scan, for example an X or a Y deflection amplifier.

Where the beam is scanned in two different directions at widely different speeds, for example at line and frame frequencies in such television apparatus, separate focusing elements associated with the different scans are employed in order to obviate any difliculty which may be experienced in applying drives of widely different frequencies, for example of different orders of magnitude, to the same focusing element.

An embodiment of the invention will now be particularly described by way of example with reference to the accompanying drawing, which is a diagram of the circuit driving a focusing element, in the form of a coil, effective to produce focusing of an electron beam onto the fiat face of a cathode ray tube.

In an embodiment described, the beam is scanned across the face of the tube in two substantially mutually perpendicular directions by a combination of scanning generators and scanning coils well known in the art. The beam simultaneously is brought to a sharply focused condition onto the tube face by means of a current applied to focus coils surrounding the neck of the cathode ray tube in a manner also well known in the art, the quiescent focus condition being considered to occur with the beam occupying a central position producing a spot in the centre of the screen.

With a constant focus field, defocusing of this beam would normally occur when the beam is displaced from this central position. To overcome such defocusing, the position of the beam at any instant is derived in the X and Y co-ordinate directions by producing position indicative signals respectively from the line and from the frame scan generators and varying the focus field in response to these position signals and according to a law dependent inter alia upon the shape of the tube face.

In the present case of a fiat face cathode ray tube, the focus field and thereby the focus current required to maintain the beam accurately focused independently of its position is approximately proportional to KX where Z is the instantaneous displacement of the beam from said central position in the tube and K is constant. Where line and frame scanning take place in two directions at right angles the focusing current may be expressed as the sum of the two components. K X +K Y where X and Y are the instantaneous displacements of the beam from said central position in the tube in the X and Y co-ordinate direction and K and K are constants.

In order to produce the K X component of the focus field, a signal derived from the line scan generator is applied to the input terminals 2 of the circuit shown in the figure, by way of a series resistance (not shown). In the section of the circuit indicated generally at 4, the input signal is reduced to its modulus by full Wave rectification at the transistors TR and TR The transistor TR is connected as an emitter follower with respect to the input voltage and in the quiescent state corresponding to zero input voltage, serves to compensate for temperature effects on the base-emitter junction of the transistorTR which at such a time is in its bottomed condition. For a non-zero input voltage the emitter of transistor TR follows the input voltage but is below it bythe transistor base-emitter voltage. The emitter voltage of the transistor TR is therefore equal to the input voltage at the terminals 2. For positive values of the input voltage, the collector of the transistor TR is inverted relative to the input whereas, for negative values of the input voltage, the collector of the transistor T R; follows the input voltage. Thus the output of the section 4, taken from the collector of the transistor TR varies as the modulus of the input voltage at the input terminals 2.

The output from the section 4 is applied to a signal squaring section consisting of a non-linear amplifier including transistors TR TRg, TR and TR and indicated generally at 6. The characteristics of the section 6 are arranged to effect squaring of the signal and to pass this squared signal to an output section indicated generally at 8 and including transistor TR Transistor TR applies an output current to an X focus coil 10 which effects focus correction and maintains constant focus conditions as a result of beam displacement in the X direction from the said central position.

In the quiescent condition, the transistors TR and TR are switched off and the transistor TR connected as an emitter follower with respect to the output of the section 4, serves to compensate for temperature effects at their base-emitter junctions. At such a time, the transistors TR and TR receive a constant predetermined direct current voltage at their bases and conduct to the maximum extent. The emitter of the transistor TR follows a non-zero output from the section 4 and is below it by the transistor base-emitter voltage. The transistor TR remains non-conducting for small signals from the section 4 but, as the signals rise above a certain level, rovides an output which is linearly related to the output of the section 4. The transistor TR operates similarly to the transistor TR but does not conduct until a higher level of the output from the section 4 is reached. The output from the collector of the transistor TR comprises the constant, or standing, direct current voltage backed off by the signals taken by the transistors TRg and TR This output voltage is applied together with the standing direct current voltage to the base of the transistor TR which converts this voltage at its base to a proportional current through the X focus coil 10.

The sum currents from the transistors TR and TR bear a proportionality to the input current at the input terminals 2 as determined by the modification, at different voltage levels and different gains in the transistors TR TR, and TR of the corresponding full wave rectified signal. (As described above, the gain of the transistor T R is zero.) Due to non-linearity of the transistor characteristics when almost turned off, the gain changes merge to some extent and the resulting signal from the transistor TR is a reasonable approximation to the square of the input signal at the terminals 2.

A similar circuit arrangement is effective to process the signal derived from the frame scan generator to produce the K Y component of the focus current which is applied to a second focus coil 10 (not shown). The combination of coils 10 and 10 together are thus effective to maintain sharp focusing of the beam irrespective of its position in the tube and therefore its position of incidence upon the tube face. I

While separate focus coils and their associated circuits have been described it will be appreciated that the signals K X and K Y may, in suitable cases, becombined and applied to a single focus coil.

In an alternativerembodiment, a single circuit may be used to process the position indicating signals derived from the scan generators. In this case, where separate focus coils are to be used, a crossover network may be employed to separate from the circuit output signal those widely differing frequency components used respectively tially of the tube face.

The invention is also applicable to a cathode ray tube arrangement in which, in addition to being scanned, the electron beam is deflected in response to an input signal indicative for example of a waveform to be displayed. In this case the electric circuit 'or circuits driving the focusing element or elements are arranged to be responsive to position signals derived from the display deflection amplifiers as well as to signals derived from the scan generator or generators if any.

I claim:

1. Apparatus for controlling focussing of an electric beam directed against the screen of a cathode ray tube furnished with focussing means, the configuration of the tube being such that the beam requires different degrees of focussing at different parts of said screen comprising a rectifying section arranged to be responsive to applied signals representative of the position of the beam within the cathode ray tube and operative to provide an output which is a modulus of the applied signals, a signal squaring section arranged to receive the output from the rectifying section and provides a signal which is approximately the square of the applied signals and an output section arranged to receive the output from the squaring section and combine said output with a direct current voltage to provide an output signal of a value such as to provide the requisite degree of focussing for each position of the cathode ray tube.

2. Apparatus as claimed in claim 1 in which the rectifying section incorporates two series-connected transistors the first of which is arranged to act as an emitter follower for the applied signals and provide an output signal having a voltage lower than that of the applied signal by the transistor base-emitter voltage, and the second transistor is arranged to receive the input signal from the first transistor and provide an output signal having a voltage equal to that of the applied signal, the output signal having a polarity inverted with respect to that of the applied signal for one polarity of said signal, and the same polarity as that of the applied signal when the applied signal is of the opposite polarity so that the applied signal is rectified.

3. Apparatus as claimed in claim 1 in which the signal squaring section incorporates a plurality of parallelconnected transistors arranged to be fed with the output from the rectifying section and become sequentially conducting as the output from said first transistor increases in power, the voltage levels and the gains of the transistors being chosen to provide that the output of the section is approximately equal to the square of the output from the rectifying section.

4. Apparatus as claimed in claim 1 in which the output section includes transistor means-operative to receive the output from the squaring section andcombine said output with a direct current voltage whereby to provide an output focussing signal of the requisite value for providing correct focussing of the beam of the cathode ray tube at each point corresponding to that defined by the input signal to the apparatus.

5. Apparatus for controlling focussing of an electron beam directed against the screen of a cathode ray tube and capable of receiving a variable signal representative of the displacement Z of the beam of the cathode ray tube from its central position in which it produces a spot in the centre of the screen and operative to provide an output signal having substantially the value KZ where K is a constant, comprising a rectifying section arranged to be responsive to applied signals proportional to Z and operative to provide an output which is a modulus of the applied signal, a squaring section arranged to receive the output from the rectifying section and provide a signal approximately proportional to Z and an output section arranged to receive the output from the squaring section and multiply said output by a factor the value of which is such that the output signal from the output section has substantially the value KZ ready to be applied to focussing means associated with the cathode ray tube.

6. Apparatus as claimed in claim 5 in which the rectifying section incorporates two series-connected transistors the first of which is arranged to act as an emitter follower for an applied signal and provide an output signal having a voltage lower than that of the applied signal by the transistor base-emitter voltage, and the second transistor is arranged to receive the input signal from the first transistor and provide an output signal having a voltage proportional to Z, the output signal having a polarity inverted with respect to that of the applied signal for one polarity of said applied signal, and the same polarity as that of the applied signal when the applied signal is of the other polarity so that the applied signal is rectified.

7. Apparatus as claimed in claim 5 in which the signal squaring section incorporates a plurality of parallel-connected transistors arranged to be fed with the output from the rectifying section i.e. with a signal proportional to Z, and become sequentially conducting as the output from said rectifying section increases in power, the voltage levels and the gains of the transistors being chosen 6 to provide that the output of the section is approximately proportionate to Z 8. Apparatus as claimed in claim 5 in which the output section includes transistor means operative to receive the output substantially proportional to Z from the squaring section, the gain of the transistor means being determined so that the value of the output from the output section is substantially equal to KZ 9. Apparatus as claimed in claim 5 in which the output signal having the value KZ consists of the sum of two components K X +K Y each component being derived from separate input signals proportional to X and Y the respective displacements of the beam from said central position in the X and Y co-ordinate direction and K and K are constants, the apparatus in corporating two rectifying sections, one to receive the X signal and one to receive the Y signal, two signal squaring sections, one to square the X modulus and the other to square the Y modulus received from the respective rectifying sections, and two output sections, one arranged to receive the squared X signal and multiply it by a factor such that the output is equal to K X and the other arranged to receive the squared Y signal and multiply it by a factor such that the output is equal to K Y these output signals being ready for applying separately to the focussing means of the cathode ray tube or for combining before applying to said focusing means.

References Cited UNITED STATES PATENTS 2,459,602 1/1949 Ulman 315-31 3,084,276 4/ 1963 Severin 315-31 3,113,237 12/1963 Schopp et a1 3l531 3,177,396 4/1965 Brooks 31531 3,412,281 11/1968 Richards et al 3l5--31 X RODNEY D. BENNETT, JR., Primary Examiner HERBERT C. WAMSLEY, Assistant Examiner 

